New in Symfony 4.3: Improved the NotBlank Validator

The NotBlank constraint of the Validator component checks that a value is
not false, null, an empty array or an empty string. Most of the other
constraints ignore null values, but NotBlank validates them. This causes
issues in scenarios such as APIs called from front-end code, where is easier
to include null fields instead of removing those fields when making requests.

In Symfony 4.3 we've improved the NotBlank constraint adding a new
allowNull option to it. By default this option is false, to keep the
current behavior. If you set it to true, then null values will be
considered valid instead of triggering a constraint violation: